I recently rebuilt my system with the case completely open and upside down.
CPU: Q9550 (E0 stepping) (8.5 x 400 = 3.4 GHz, 1.2V)
CPU heatsink: Thermalright Ultra 120 Extreme
Chipset heatsink: Thermalright HR-05 IFX
Main drive: OCZ Vertex 60GB SSD
Storage drive: WD 1TB GP
Mainboard: Gigabyte EP45-UD3L
RAM: G.Skill PC2-8000 2 x 2GB (400MHz, 4-4-4-12, 1.8V)
Video: PNY GTS 250 1GB (with Arctic Cooling Accelero S1 Rev. 2)
PSU: Silverstone Nightjar ST45NF
Fans: 2 x Noctua NF-P12; 1 x Noctua NF-R8 (all at 800rpm)
The result is not, I concede, pretty to look at. But I'm sold on the thermal advantages of removing the PSU from the case, and the loss of sound damping from having the case open is more than offset by the slower fan speeds I'm able to use.
I'm running my fans slower and the CPU temps still dropped 10 deg at 100% load from the previous upright position. The system is always at full load, since I run BOINC projects on both the CPU and GPU. CPU temps are in the high 40s and GPU temps are in the 60s. The space beneath the CPU cooler is completely open.
With the fans at 800rpm, the only noise audible sitting at the desk is the HDD when it spins up, which is rare since I use the SSD for the OS and applications.